Duets for violin and viola may seem a rather obscure branch of the chamber music family, but they were quite popular in 18th-century Austria. Joseph Haydn wrote a set of six sonatas for the soprano and alto instruments in 1773, and ten years later the Archbishop of Salzburg - the imperious Hieronymous Colloredo who made Mozart (1756-1791) miserable - ordered Haydn's brother Michael to compose another set of six.

Michael Haydn was able to complete only four of the required six, however, before illness intervened. Misinformed or unconcerned about Haydn's situation, the Archbishop threatened to stop the salary of this distinguished musician who had been in his employ for 20 years. The younger Haydn, a composer who is only beginning to get the recognition and performances he deserves, was much admired by Mozart and a close personal friend. When Mozart heard of his colleague's plight in the summer of 1783, he quickly wrote the two delinquent duos himself and delivered them to the Archbishop under Haydn's name.

Mozart did make some effort to imitate elements of Michael Haydn's style. That is apparent in the bird song trills and grace notes that decorate the opening movement of K. 424, the slow dance form of the middle movement, and the form of the finale, a theme and variations movement of the sort that Joseph Haydn had developed and had used in his own duets.

But this graciously virtuosic piece - like its companion in G major, K. 423 - is unmistakably Mozartean. The chromaticism of the slow introduction to the first movement and the tautly argued development of the ensuing Allegro, the expressive poignancy of the slow movement, which gently rocks in the meter of the old siciliano dance, and the sparkle of the variations all attest to the true identity of the composer, homage to his friend notwithstanding. Mozart played both violin and viola well, and this is - like the Sinfonia concertante, K. 364 - an equal partnership in terms of both musical expression and technical demands.

-- John Henken is the Los Angeles Philharmonic's Director of Publications.